Sensing strips are widely used on the edges of power doors to provide safe power door operation. A person or object in the door closure path causes the strip to deform and an electrical contact to close, resulting in immediate deenergization or reversal of the drive motor. Thus injury to the person or object obstructing closure of the door is prevented.
One type of sensing strip, disclosed in U.S. Pat. No. 3,315,050 to Miller, utilizes a pair of foam pads carrying conductive aluminum sheets. Intermediate the foam pads is another foam strip formed with openings. Upon deformation of the outer foam pad, the aluminum sheets are forced into contact through one of the openings to close a suitable circuit.
To overcome the operating problems occurring with the Miller type strip when the strip jacket is ruptured by vibration or puncture, permitting moisture and contaminates to enter, an improved power door sensing strip impervious to moisture and contaminants and reliably operative when engaged by an object, is disclosed in U.S. Pat. No. 5,087,799 by the applicants of this invention. In that sensing strip, a pair of electrical conductors are positioned in a sealed plastic tube that is supported in a molded foam block so that external forces result in contact between conductors. To ensure reliable operation, there are no internal supports placed between the conductors. Moreover, short circuits caused by moisture and contaminants entering the foam, in the event an outer jacket seal is broken, are eliminated.
While the invention of U.S. Pat. No. 5,087,799 provides a sensing strip with many advantages, the cost of such a sensing strip inhibited sales to residential customers. In particular, photoelectric cells mounted about six inches above a door sill can be sold at low cost, thereby providing a cost advantage over the sealed safety sensing strip. However, it is desirable to have a sensing strip closed by deformation since the door can then be powered to one inch above the sill, rather than six inches, before being driven closed by the motor operator. Hence it would be advantageous to have a safety sensing strip competitive economically with photo cells, for use on residential power operated garage doors.